As the world demand for data continues to increase, more data is transmitted optically through indoor/outdoor cables using optical fibers, to deliver fiber to the home (FTTH), within the data center, central office or enterprise environments, as well as in fiber fed backhaul applications for wireless transmission or fiber to the antenna (FTTA) applications. These applications demand low cost, reliable methods to join and terminate the ends of optical fiber cables.
In the situations where test access or reconfiguration is required, ferrule based connectors such as SC, LC, MT format optical fiber connectors will be used, due to their durable construction. On the other hand, permanent joints or splices are used to join optical fibers where the lowest optical loss is required. Conventional optical fiber splicing technologies include fusion splicing and mechanical splicing.
Fusion splicing utilizes an arc to fuse or melt the ends of two optical fibers together. The splicing machines are expensive ($3,000-$10,000), fragile instruments, operated by specially trained technicians. Proper use, results in a reliable low optical loss joint. Fusion splicing is especially attractive where large numbers of fibers need to be spliced at a given location. However, it becomes cost prohibitive to equip thousands of technicians with fusion splicers as they construct FTTH links to individual subscribers.
Mechanical splice uses a mechanical structure to align and clamp two optical fiber ends, resulting in a low-cost installed splice. It can be challenging to prepare and mate optical fiber ends in a mechanical splice and have intimate glass to glass contact every time. For example industry standard cleavers deliver +/−1 degree cleave angle on the end face of an optical fiber. When two cleaved fibers are slightly angled and mated in a mechanical splice, a small air gap can occur between the active portions of the optical fibers. In order to minimize reflection from the glass-air-glass interfaces, an index match gel is used at the fiber joint to enhance the optical performance of mechanical splices.
Common mechanical splices use substrate materials with a higher coefficient of thermal expansion (CTE) greater than the glass (silica) in the optical fibers core and cladding. Substrate examples are polyether imide materials and aluminum. CTE mismatch during thermal loading can cause the gap between the fiber tips to change with temperature, which can place demand on the index matching gel to flow and fill this variable gap.
Typical index matching gels are a thixotropic blend of silicone oil loaded with micron sized fumed silica particles. The index matching gel is selected to match the index of refraction of silica at room temperatures. Their thixotropic nature allows the index matching gels to be sheared so that the can be dispensed into the mechanical splice, but prevent the gels flowing or wicking out of the gap between the ends of the optical fibers. The index matching gels have a higher dn/dT (i.e. the change of refractive index with temperature) than silica resulting in refractive index mismatch at temperature extremes. This index of refraction mismatch at the fiber to gel interface, causes a predictable, repeatable, slight reflection signal response that varies with temperature.
An opportunity exists for a method of permanently joining the ends of optical fibers with an index matched, a low dn/dT visible light cure optical coupling adhesive.